Improved vehicle fuel economy is both a consumer expectation and a regulatory requirement. For example, federal regulations have been implemented that require 40% improvement in fuel economy for passenger cars and light trucks. One approach to achieving such dramatic improvements has been to reduce engine size. Engine size reductions have been disproportionately greater than vehicle weight reductions and aerodynamic improvements, thus resulting in under-powered vehicles and poor vehicle performance. Some engine power can be recovered by boosting engines with turbochargers and/or superchargers. However, boosting smaller engines with turbocharging commonly results in turbo lag, which is the delay between the demand for increased power (opening of the throttle) and the turbocharger providing increased air pressure and enhanced engine power. Traditional turbochargers that receive exhaust gases from multiple cylinders in an engine can be configured to reduce turbo lag to acceptable customer expectations (e.g., as is typically done by restricting a turbocharger turbine inlet nozzle, implementing a variable geometry inlet nozzle in a diesel engine, or multiple turbine turbochargers that utilize exhaust gases aggregated from multiple combustion chambers), but this has come at the expense of high RPM performance in traditional turbocharged engine configurations—that is, immediate turbo response at tip in has meant less power at high RPMs. Putting a smaller nozzle on a turbocharger turbine inlet effectively increases exhaust gas velocity, but causes a substantial increase in back pressure, which degrades fuel economy by creating parasitic exhaust pumping work and elevating engine temperatures, which reduce combustion efficiency and component reliability.
Under turbo lag acceleration conditions, driver torque demand forces fuel air mixtures to become very rich to achieve higher engine torque output, which leads to degraded fuel economy and emissions (optimum fuel and emissions are normally closed-loop-controlled at stoichiometric, 14.6 to 1.0 air-fuel ratio). It is common during acceleration in current engines for air to fuel ratios to drop to a range between 12.0 to 1.0 and 9.0 to 1.0 to improve acceleration. These rich air-fuel ratios reduce fuel economy and increase emissions.
Turbochargers have two essential components:                a turbine wheel that harvests energy from engine exhaust gas, and        a compressor wheel that is connected to and driven by rotation of the turbine wheel.        
The compressor delivers fresh air to the engine creating pressure and increasing induction charge density under which engine power will increase in proportion to the elevated charge density. Both components of the turbocharger are centrifugal pumps that achieve high efficiency at high rotational speeds (e.g., 100,000 RPM to 300,000 RPM, depending on turbine wheel diameter and maximum engine output). At low RPMs, the turbine wheel and compressor wheel outputs are low, providing a slow-rising pressure curve of increased charge density and a corresponding slow-rising engine power output. The above is most evident when a vehicle accelerates from a standing start, producing noticeable turbo lag during such transient acceleration conditions. Turbocharger systems to date (also referred to as “earlier systems” and the like) have collected exhaust gases from a number of cylinders in a manifold or similar exhaust gas collecting or aggregating structure and directed the collected gases to a single turbine situated significantly downstream of the exhausting cylinders.
Each combustion chamber of an engine is designed to accelerate piston movement as a result of controlled explosive combustion. The process is approximately 25% efficient; the remaining 75% is thermally disbursed to the cooling system and the exhaust system. Transfer of energy to an engine's exhaust system is driven by combustion pressure, the energy being released through each combustion chamber's exhaust valve(s). At low engine loads, and immediately before the exhaust valve opens, combustion chamber pressure is high and pressure outside the exhaust port is low. When the exhaust valve opens, the high pressure differential between the combustion chamber and the exhaust port creates a high velocity pulse. In traditional engine configurations, the high velocity pulse is dissipated by a number of factors (e.g., the volume of the exhaust manifold, turns and distance in the exhaust gases' path between the exhaust valve and the turbine wheel entrance due to multiple cylinders and multiple exhaust runners feeding a single turbine wheel. The pulse can also be dissipated in some engines (e.g., four cylinder and five cylinder applications) due to interference between exhaust gases from two cylinders whose exhaust discharges overlap.
Overview
Implementations of an internal combustion engine include a single turbocharger mounted to each combustion cylinder to provide improved harvesting of exhaust gas energy. In some implementations each turbocharger is mounted in close proximity to its associated combustion cylinder to provide improved harvesting of exhaust gas energy due to the short exhaust gas travel distance, small volume, and minimal turns between the exhaust valve and the turbine wheel. Dynamic engine charge boosting is improved, as is the harvesting of exhaust gas energy (including harvesting energy during pulse exhaust gas conditions, which has been lost in traditional turbocharging systems due to those systems' mixing of multiple cylinders' exhaust gases, long exhaust gas travel distances, larger volumes, and prevalent bending and turns in the path between the combustion cylinder exhaust valve and the turbine wheel). Additionally, a single, smaller turbocharger per cylinder as disclosed and claimed in some implementations herein permits dramatic reduction in the moment of inertia (i.e., rotational inertia) to be overcome by the turbine and compressor wheels (in some cases by more than 50%) as compared to one large turbocharger collecting multiple-cylinder exhaust gases—the improved rotational inertia characteristics are due to the smaller diameters and masses of the turbines and compressor wheels used in the disclosed and claimed implementations. As a result, a single turbocharger per cylinder can provide improved engine response with less restriction of the turbocharger inlet, substantially reducing or eliminating turbo lag time, while maintaining an efficient air-fuel ratio—that is, engines equipped with such implementations exhibit no lag, better fuel economy and reduced exhaust emissions.
Traditional approaches to turbocharging utilize a single turbocharger sized to serve multiple cylinders, requiring high rotational inertia turbine and compressor wheels with no dynamic opportunities to increase boost at low RPM and/or light load conditions. Additionally, as noted above with regard to such traditional approaches, because of runner length, larger volume, additional turns and added friction there is no practical way for a single turbocharger serving multiple cylinders to harvest exhaust pulse energy—the turbocharger is too far away from the cylinders both physically and functionally to permit such harvesting.
In addition to the quicker turbocharger response, multiple compressors (one in each cylinder's associated turbocharger) permit alternate modes of compressor output configuration. “In series” compressor boosting can be configured so that one compressor feeds compressed air to the input of the next turbocharger's compressor, for example for low load conditions. In an exemplary 3-cylinder engine, boost pressure thus can be tripled at low loads. Once the initial vehicle launch is achieved, the serial compressed air configuration can be reconfigured to parallel operation (e.g., through controlled valving of the compressors' outputs to an intake air manifold or the like). Additionally, under part throttle conditions, “in series compressor air boosting” can raise torque by 200% to 300% (depending on the number of cylinders), allowing earlier shift schedules, which improves fuel economy. Thus, interconnection of compressors as disclosed, taught and claimed in some implementations herein permits (1) “in series” centrifugal compressor air boosting that increases charge density at low RPMs and/or light loads (e.g., where a second compressor is additive to a first compressor and a third compressor is additive to the second compressor), and (2) parallel compressor alignment that provides charge density control under medium to high RPMs and/or load conditions.
The single turbocharger per cylinder engine configuration also allows for a larger turbine intake port (i.e., no restricted nozzle size as in earlier turbocharger systems), which reduces exhaust back pressure and lowers cylinder head temperatures, thereby improving fuel economy. In four and five cylinder engines, the single turbocharger per cylinder eliminates blow down interference and the loss of efficiency experienced in dual scroll turbochargers.
A single turbocharger per cylinder engine implementation can eliminate detectable turbo lag during acceleration from a standing start. It also allows closed loop stoichiometric fuel control (i.e., maintaining the 14.6 to 1 air-fuel ratio) under a wider range of operating conditions, providing improved fuel economy and emissions. Further, under transient acceleration conditions, implementations disclosed herein greatly extend and maintain stoichiometric fuel control (i.e., 14.6 to 1 air-fuel ratio) providing improved fuel economy and emissions characteristics.
Systems, methods, apparatus, and software for turbocharging an internal combustion engine are provided herein. In one example, a turbocharger is affixed (in some implementations in close proximity to) the exhaust valve(s) of each cylinder (or other combustion chamber). Each turbocharger has a turbine having an inlet configured to receive high energy exhaust gas from the combustion chamber to which the turbocharger is affixed. Each turbine drives a compressor that receives fresh air (which can be previously compressed air or not) and pressurizes that incoming fresh air before it is discharged via the compressor's outlet. The outputs of several turbochargers' compressors can be configured in series relative to one another's compressor inlets to compound pressurization of fresh air before it is delivered to an intake manifold or the like, or can be configured in parallel so that each compressor delivers its output compressed fresh air directly to the intake manifold. Wastegates can be provided for each turbocharger to permit bypassing the turbocharger's turbine under certain conditions.
In some implementations balancing valves can be provided in lieu of (or in addition to) wastegates and can be configured to provide cross-flow delivery of exhaust gas from one cylinder to the turbine inlet of a different cylinder's turbocharger turbine. Such cross-flow delivery can assist in balancing exhaust gas flow through the turbochargers and reduce turbulence. Aspects and components of the various implementations can be controlled by a control system such as an engine control unit or the like. Use of one or more balancing valves interconnecting turbine inlet passages (a) allows switching from or transitioning between exhaust gas pulse flow with little or no cross-flow between turbochargers (e.g., at low RPMs and/or light loads) to constant pressure flow with greater cross-flow between turbochargers (e.g., for improved stability at high loads), and (b) can provide a conduit to a single wastegate that serves all cylinders to control maximum boost pressure.
The various implementations provide advantages over other internal combustion engine turbocharging systems, including (either alone or in combination):                enhanced harvesting of combustion chamber exhaust gas energy due to the closer proximity of individual turbochargers affixed near each combustion chamber's exhaust valve(s);        faster turbocharger response due to reduced rotational inertia of smaller turbocharger component sizes (e.g., for turbines and compressor wheels);        larger turbine inlet sizes that permit better performance at high RPMs and reduce exhaust gas back pressure when encountering turbocharger turbines;        lower combustion temperatures and power head component temperatures resulting from reduced back pressure, which improves fuel economy and maintenance and repair of parts;        maintenance of air-fuel ratios throughout a wide range of engine speeds;        improved fuel economy in transient acceleration conditions and changing power demands.        